My own personal experience with my four-year-old Blackbook (which remains plugged in 98% of the time and is only drained/calibrated maybe 3-4 times a year) is that the original battery now gives me about 60% of its original capacity (135 cycles just for the record). This is exactly on par with where this battery should be for its age (ie loses about 10% per year on average).

I *did* calibrate it (ie charged it fully, ran it to empty, charged it fully again) the first time I used the machine, but as I say I haven't done so regularly ever since, and most of the time it's on my desk plugged in.

So as you can see, mileages vary. As long as you calibrate from time to time (per Apple's guidelines, which are not laws or rules just *guidelines*), you should get as much life from the battery as the technology is capable of giving you.